1. Field of the Invention
The invention relates to an image forming apparatus which can adjust a density.
2. Related Background Art
FIG. 4 is a circuit block diagram showing an example of a development density adjusting circuit according to a conventional image forming apparatus. Reference numeral 100 denotes a high-voltage power source control unit to control a high-voltage power source output to a photo sensitive material and 101 indicates a volume resistor. By making a resistance value of the volume resistor 101 variable, a high-voltage DC output PriDC to determine a charging potential to the surface of the photo sensitive material and a high-voltage DC output DevDC to decide a development bias potential are interlocked and each of the above potentials can be linearly adjusted.
Although the high-voltage power source output includes a copy transfer high-voltage output or a high-voltage AC output which is multiplexed to the high-voltage DC output PriDC in dependence on a system processing format or the like in addition to the above outputs, they are omitted here.
In the high-voltage power source control unit 100, OSC denotes a pulse output which is supplied to transistors 109 and 129 to drive transformers 118 and 130 to generate the high-voltage DC output PriDC and the high-voltage DC output DevDC. HV.sub.1 denotes an ON/OFF signal of the high-voltage DC output PriDC and HV.sub.2 indicates an ON/OFF signal of the high-voltage DC output DevDC.
The high-voltage DC outputs PriDC and DevDC are determined by emitter potentials of transistors 116 and 145, namely, by outputs of operational amplifiers 113 and 140, respectively. Further, the outputs of the operational amplifiers 113 and 140 are decided by constructions of the volume resistor 101, resistors 102 104, and 105, and a power source voltage VCC. That is, the outputs of the operational amplifiers 113 and 140 are decided by an open loop gain due to voltages to inverting inputs of the operaitonal amplifiers 113 and 140 and differential voltages to non-inverting inputs thereof. Since the voltages to the inverting inputs and the non-inverting inputs of the operational amplifiers 113 and 140 are almost equal, the high-voltage DC output PriDC is determined by a voltage drop component through a resistor 126 by a current ipri which is caused due to a difference between the input voltage to the inverting inputs and the voltage which is obtained by dividing the power source voltage VCC by resistors 122 and 123. The same shall also apply to the high-voltage DC output DevDC.
Reference numerals 112, 115, 120, 133, 137, and 141 denote capacitors; 114, 117, 119, 131, 132, and 146 diodes; 103, 106, 107, 108, 110, 111, 121, 124, 127, 128, 134, 136, 138, 139, 142, and 144 resistors; and 125 and 143 transistors.
The development density adjusting operation by the volume resistor 101 will now be described hereinbelow with reference to FIG. 5.
FIG. 5 is a diagram showing the relation between the development density adjustment by the volume resistor 101 shown in FIG. 4 and the adjustment potential. An axis of ordinate denotes a potential and an axis of abscissa indicates an adjustment level F. F.sub.1 corresponds to a GND potential, F.sub.5 corresponds to VCC/2, and F.sub.9 corresponds to VCC.
As will be understood from the diagram, each time the user operates a density setting switch in an operation unit, the adjustment level F changes and the high-voltage DC output PriDC and the high-voltage DC output DevDC which decides the development potential are linearly made variable in accordance with such a level change, so that a print image can be adjusted from a dense state to a light state. FIG. 5 shows the characteristics in the case where the electrophotographic process is performed by an image exposure, namely, the case of an inversion development in which an exposing portion is developed.
In the above conventional apparatus, however, the toner density adjustment is unconditionally determined by only the volume resistor 101, so that there are the following problems.
That is, an information which is processed by the image forming apparatus, not only a text image but also a graphics image can be processed. In the text image, as a language which is processed, not only Japanese but also English or European characters and the like can be processed. Further, a resolution can be also arbitrarily set (for instance, 200, 300, 400, 600 dpi (dots per inch)). In the case of such a flexible construction, one density adjustment level can be varied only by a default value, the density adjustment which is optimum to the output information cannot be performed, and the print quality extremely fluctuates.
If the user sets the corresponding density level, considerable temporary outputting processses are needed until an output of the optimum density is derived. There are problems such that the running costs multiplicatively increase and the like.
Hitherto, the density level can be adjusted only every level by the volume resistor 101 in spite of the fact that the toner density changes also for a change in processing condition due to an environmental fluctuation. Thus, there are problems such that it is necessary to execute the density setting scanning operations many times until the optimum toner density is obtained, unnecessary test printing processes and the like are required, an economical burden increases, and the like.